1. Field of the Invention
The present invention relates to a normal conducting type electromagnet for bending a charged particle beam, particularly an electromagnet adapted for use in a synchrotron radiation beam (hereinafter called SR beam) generator.
2. Description of the Related Art
An SR beam generator radiates SR beams from predetermined positions by accelerating electrons (or positrons) along a predetermined orbit to near the light speed. Various types of SR beam generators have been proposed. There is a strong need of a compact SR beam generator. An SR beam generator having an orbit radius of about 0.5 m has been in a practical use.
FIG. 11 is a schematic diagram showing the structure of an SR beam generator of a racetrack type using an electron storage ring. A pair o-f semicircular orbits having a curvature R are formed by two bending electromagnets 51a and 51b. The pair of semicircular orbits are coupled by two straight orbits to form a racetrack type orbit 50 in a vacuum container. Disposed along the straight orbits are four first quadrupole electromagnets 52a, 52b, 52c, and 52d, four second quadrupole electromagnets 52a, 52b, 52c, and 53d, an RF accelerator cavity 54, and an incident beam kicker electromagnet 55 disposed at an electron beam input position.
An electron beam generated by an injection beam accelerator (not shown) is introduced from the electron beam input position into the vacuum container, accelerated and deflected to have a preset curvature respectively by the RF accelerator cavity 54 and the bending electromagnets 51a and 51b to circulate the beam along the orbit 50 at near the light speed.
Examples of conventional bending electromagnets are shown in FIGS. 12A and 12B, and 13A and 13B.
FIG. 12A is a partial plan view of a conventional bending electromagnet, and FIG. 12B is a cross sectional view of the electromagnet taken along one-dot chain line B12--B12 shown in FIG. 12A.
As shown in FIG. 12A, a coil 63 is wound around a pair of arc pole pieces 61.
As shown in FIG. 12B, a gap 64 defining part of an electron orbit Is formed between the pair of pole pieces 61. A yoke 62 surrounds the pole pieces 61 and coil 63 to form a magnetic circuit 65 constituted by the yoke 62, pole pieces 61, and gap 64.
A magnetomotive force required by a coil increases rapidly if the magnetic flux density greater than the saturation flux density of the pole pieces is to be obtained. The bending electromagnet having the structure and shape shown in FIGS. 12A and 12B has a largest magnetic flux density at the pole pieces 61 near the yoke 62 so that as the magnetomotive force is increased, the magnetic saturation occurs first at this area.
In the bending electromagnet shown in FIGS. 13A and 13B, a coil is wound also around a gap as different from the electromagnet shown in FIGS. 12A and 12B. FIG. 13A is a plan view of the bending electromagnet, and FIG. 13B is a cross sectional view taken along one-dot chain line B13--B13 shown in FIG. 13A.
Pole pieces 71, a yoke 72, a coil 73a, and a gap 74 have the similar structures as the pole pieces 61, yoke 62, coil 63, and gap 64 shown in FIGS. 12A and 12B. A coil 73b is wound also around the gap 74 as different from the bending electromagnet shown in FIGS. 12A and 12B. The coil 73b functions to increase a magnetomotive force and to improve a uniformity of a magnetic field distribution in the gap 74. In order not to be an obstacle of the electron orbit, the coil 73b is curved and bent down or up at opposite ends of the gap 74 in the circumferential direction.
This arrangement shown in FIGS. 13A and 13B is particularly effective for an electromagnet having a large gap 74. However, an SR beam is not radiated from the electromagnet of this type so that this electromagnet cannot be used as an electron storage ring.
There are superconducting and normal conducting bending electromagnets. Although the superconducting bending electromagnet can generate a strong magnetic field, the system using this electromagnet becomes bulky and complicated because of related apparatuses. Furthermore, a highly sophisticated manufacturing technique and a large number of manufacturing processes arc required, resulting in a high cost.
The saturated magnetic flux density of iron forming a normal conducting electromagnet is in the order of 2.15 teslas at most. If a magnetic flux density of 2.15 teslas or higher is to be generated, a required magnetomotive force increases rapidly. From this reason, the normal conducting electromagnet has been generally used at 2.15 teslas or lower.
The orbit radius of a bending electromagnet of an SR beam generator is determined by a magnetic field. The stronger the magnetic field, the smaller the orbit radius. This magnetic field intensity constraint has made it more difficult to provide a compact electron storage ring of a normal conducting bending electromagnet than a superconducting bending electromagnet.